Human granulocyte colony-stimulating factor (G-CSF) is a glycoprotein containing 204 amino acids with 30 amino-acid signal peptides. Mature G-CSF protein, having a molecular weight of 18-20 kDa, is composed of 174 amino acids without signal peptides and secreted out of the cells. Human cells mainly responsible for such secretion are monocytes, fibroblasts, and endothelial cells.
There are three main biological functions for G-CSF in vivo, namely:
acting on neutrophil precursor cells and bone marrow stem cells. Actuating the polarization, proliferation and maturation of neutrophilic granulocyte.
activating the mature neutrophilic granulocyte to participate in immune response, and cooperating with other hematopoietic growth factors, such as Stem Cell Factor, Flt-3 ligand, and GM-CSF to exert hematopoietic functions.
G-CSF Receptor (G-CSFR) is proven to exist mainly in bone marrow hematopoietic stem cells Sca+Lin-Th1low, precursor cells CD34+, committed granulocyte precursor cells, and mature neutrophils. G-CSFR is a specific receptor having a high affinity to G-CSF and is composed of 812 amino acids.
Tamada et al. obtained the crystalline structure of the G-CSF: G-CSFR complex and the stoichiometry of G-CSF: G-CSFR complex was shown as a 2:2 ratio by the 2.8 angstrom diffraction analysis (PNAS, 2008, Vol. 103: 3135-3140), i.e., a complex comprises 2 ligands and 2 receptors bound together. In other words, in each complex, each G-CSF molecule binds to one receptor chain molecule; when both G-CSF molecules are bound with G-CSFRs, they are brought to close proximity and a 2:2 dimer is formed as a result of this interaction. Under this circumstance, the carboxyl terminal of the G-CSF receptor is then able to activate the downstream signal molecules Janus tyrosine kinases JAK2. JAK2 then stimulates cell differentiation and proliferation by activating STAT3 to switch on gene transcription.
In 2003, Schabitz W. R. et al. reported that recombinant human G-CSF was shown to have a protective functionality on nerve cells in the ischemic animal model (Storke, 2003, 34; 745-751). In 2006, Shyu et al. reported that rhG-CSF was shown to have clinical efficacy in the treatment of patients having acute stroke in which the patients were administered with rhG-CSF daily for five consecutive days (CMAJ, 2006, 174:927-933). The in vivo half-life of G-CSF in rat upon subcutaneous administration is about 2 hr, whereas the half-life of G-CSF in human upon subcutaneous administration is only 3.5 hr. Therefore, patients needed to be administered with the drug on a daily basis, and this affected the living quality of patients.
Neurodegenerative disease is a condition of neuronal loss in brain and spinal cord. It is a kind of chronic and progressive disease of the nervous system, mainly including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington disease, amyotrophic lateral sclerosis, spinal muscular atrophy, and spinal cerebellar ataxias, etc. These neurological diseases are characterized by a common feature of degeneration and apoptosis of neurons, which result in the abnormal behavior and dysfunction of patients, and lead to a premature death. The pathogenesis of neurodegenerative diseases remains obscure, as yet no existing effective method and medicine are available. Current treatments for PD comprise the replenishment of the substances deficient neurons in patients' brain via oral administration or intravenous injection, such as levodopa, whereas levodopa cannot efficiently control the naturally pathogenic progression of PD and cannot affect the speed of degeneration of dopaminergic neurons. Moreover, use of levodopa brings adverse side effects, such as on-off phenomenon and dyskinesia, and its therapeutic effects only last about 2 years. Long-term use of levodopa may cause neuronal damage as well as apoptosis of the neurons. Current treatments for AD comprise increasing the concentration of acetylcholine directed against the deficiency of acetylcholine in AD patients' brain. This method cannot control the development of the disease, either.
At present, medicines for treating PD are mostly to reduce symptoms, such as dopamine replacers (levodopa or dopamine agonists). Among them, levodopa (L-DOPA) supplements dopamine in the brain as a precursor of dopamine, which is the most commonly used and effective therapy for PD. However, long-term usage of such drug may easily reduce the curative effect and bring serious side effects, even an on-off phenomenon. In addition, prevention of the loss of dopaminergic neurons is also one of the main strategies for the treatment of PD, in which neurotrophic factor (GDNF) is studied the most currently. However, GDNF was not shown to exhibit efficacy but a series of side effects in the clinical trials, such as nausea, anorexia, and weight loss, etc. (Neurology, 2003, 69:69-73). Use of G-CSF in the treatment of PD has also been reported. However, in both animal models of PD and clinical trials, the administration dosage of G-CSF is high, the therapeutic response is slow, and more frequent administration and longer treatment duration are needed, resulting in a reduction of patient compliance and making it inconvenient for the patients to use the drug.
Therefore, there is an urgent need in the art to develop more effective drugs for treatment of neurodegenerative diseases.